Boundary Mixing Associated with Tidal and Near-Inertial Internal Waves

Abstract

Abstract Moorings deployed on the south (August–November 2002) and north (November 2002–June 2003) flanks of the Kaena Ridge, Hawaii, are used to document the flow variability associated with mixing within 200 m from the boundary, deep along the ridge, as part of the Hawaii Ocean Mixing Experiment (HOME). At both sites, strong temperature inversions are detected with vertical scales of ∼100 m. A Thorpe-scale analysis of the overturns yields a time-averaged dissipation near the bottom at the south site (1.2 × 10−8 W kg−1) that is 10 times higher than the north site (1.9 × 10−9 W kg−1), with both higher than the dissipation at similar depths 30 km from the ridge. On the south flank, observed horizontal currents and vertical displacements are dominated by the semidiurnal internal tide. On the north flank, the semidiurnal tide is less energetic than on the south, with a different vertical structure as tidal amplitudes decrease toward the boundary. These differences are attributed to greater separation from the bottom of downward-propagating internal tides at the north site compared to the south site, resulting in higher mixing at the south site. Near the boundary, near-inertial to diurnal oscillations are more energetic at the north than the south site. This asymmetry is attributed to near-inertial internal waves that are generated north of the ridge by winter storms; the ridge shadows the equatorward-propagating near-inertial internal waves leading to negligible amplitudes on the southern lee side. The near-inertial waves combine with internal tidal motions to create high strain conditions that lead to mixing at the north site.